This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in Japanese Patent Application No. 2001-121300 filed on Apr. 19, 2001.
1. Field of the Invention
The present invention relates to a power transmission device for transmitting power from the engine of heavy motor vehicles such as trucks and buses to the transmission, comprising a fluid coupling and a friction clutch, and more particularly to a gear parking brake enabling in-gear parking.
2. Description of the Related Art
Conventionally, a power transmission device that uses a fluid coupling in combination with a wet multiplate friction clutch has been proposed as a power transmission device for transmitting power from the engine of heavy motor vehicles such as trucks and buses to the transmission.
As shown in FIG. 5, this power transmission device has a fluid coupling 4 and a wet multiplate friction clutch 5 joined between the engine""s crankshaft 1 and input shaft 3 of the transmission 2, so as to be able to connect and disconnect. This enables smooth gear change and reliable power transmission to be achieved without it being necessary for the driver to operate the clutch.
The basic structure and application of a conventional power transmission device having this kind of structure will be described below.
First, in the same way as a well-known conventional fluid coupling, the fluid coupling 4 mainly consists of a pump shell 7 fixed to the engine""s crankshaft 1 through a casing 6, a turbine shell 8 positioned opposite the pump shell 7, and a stator 9 fixed therebetween. The torque of the crankshaft 1 is delivered from the pump shell 7 through the fluid filled therein to the turbine shell 8, and the torque of the turbine shell 8 is transmitted to an output shaft 11 that is connected to the wet multiplate friction clutch 5, through a hub 10. Further, a lock-up mechanism 13 (to be described later) activated by a fluid-activated mechanism 12 is formed between the casing 6 and the turbine shell 8, and when the engine speed reaches a prescribed speed, the crankshaft 1 and output shaft 11 are directly linked through the above mechanisms and efficient power transmission is achieved.
Next, the wet multiplate friction clutch 5 comprises a clutch outer 14 spline fitted to the output shaft 11, and a clutch center 15 mounted to the input shaft 3 of the transmission 2. In the same way as in a conventional wet multiplate friction clutch, a plurality of friction plates 16 formed on the inside of the clutch outer 14 and a plurality of friction plates 17 formed on the outside of the clutch center 15 are made to come into contact with each other, thereby transmitting the power of the output shaft 11 to the input shaft 3 of the transmission 2. Further, this wet multiplate friction clutch 5 contains a connection and disconnection mechanism 18 activated by a fluid-activated mechanism 12, which will be described later, and when the engine is started or the gears are changed, the clutch outer 14 and the clutch center 15 disengage and the power between the output shaft 11 and the input shaft 3 is cut off.
Further, the fluid-activated mechanism 12 that operates this lock-up mechanism 13 of the fluid coupling 4 and the connection and disconnection mechanism 18 of the wet multiplate friction clutch 5 mainly consists of a hydraulic pump 21 formed inside an intermediate wall 20 of a joint housing 19 that contains the above-mentioned fluid coupling 4 and wet multiplate friction clutch 5; a lock-up mechanism control valve 22 and a connection and disconnection mechanism control valve 23 activated by the operating oil discharged from the hydraulic pump 21 as shown in FIG. 6; and electromagnetic selector valves 24 and 25 to control the activation and shutoff of these control valves 22 and 23. As shown in FIG. 5, this hydraulic pump 21 operates the whole time that the engine is running, accompanying the rotation of the pump shell 7 of the fluid coupling 4, and it sucks in the operating oil that is pooled in the operating oil retention space 26 at the bottom of the joint housing 19 through a suction passage 27. The operating oil is sent to the above-mentioned lock-up mechanism control valve 22 and the clutch connection and disconnection mechanism control valve 23 and either activates these valves or after passing through is returned back into the operating oil retention space 26 in a cycle. Further, the electromagnetic selector valves 24 and 25 that activate the lock-up mechanism valve 22 and the connection and disconnection mechanism control valve 23 are each controlled by control signals from a controller consisting for example of a micro-computer (not shown in the drawings).
In a power transmission device having this kind of construction, first, when the engine starts up, the lock-up mechanism 13 of the fluid coupling 4 and the connection and disconnection mechanism 18 of the wet multiplate friction clutch 5 are both turned to a disconnected (OFF) state. Therefore, in the fluid coupling 4 a state of so-called creep occurs, the power from the engine""s crankshaft 1 is cut off by the wet multiplate friction clutch 5, and is not transmitted to the input shaft 3 of the transmission 2. Next, from this state when the driver shifts the transmission 2 into gear to move the vehicle, the controller, which receives this signal, activates the electromagnetic selector valve 25 and the clutch connection and disconnection mechanism control valve 23. Accordingly the connection and disconnection mechanism 18 is activated by the operating oil discharged from the hydraulic pump 21, and by connecting the wet multiplate friction clutch 5, the power from the output shaft 11 is transmitted to the transmission 2 through the input shaft 3 and the vehicle begins to move. In such a way, the engine""s power is transmitted to the transmission 2 and the vehicle starts moving, and when a prescribed speed is reached, the driver then operates a shift lever 28 to shift the transmission 2 up to second gear, third gear and so on in order to further increase the speed of the vehicle. However, each time the driver shifts up to a higher gear, it is automatically controlled so that the instant that the driver operates the shift lever 28 this is detected by the controller and the wet multiplate friction clutch 5 is momentarily cut off. Then, after the shift to the next gear has been completed the wet multiplate friction clutch 5 is automatically reconnected, and thus smooth upshift is achieved. Further, at the same time, when the vehicle has reached a prescribed speed, the lock-up mechanism 13 of the fluid coupling 4 is activated by an electromagnetic control valve 24, which is also activated by the controller. By directly connecting the crankshaft 1 and the output shaft 11, power from the engine""s crankshaft 1 is transmitted to the transmission 2 without loss.
However, in a vehicle with a power transmission device having this kind of construction, so-called in-gear parking cannot be achieved. In other words, when the engine 1 is stopped in order to park, the hydraulic pump 21 for engaging the wet multiplate friction clutch 5 is inactivated and the wet multiplate friction clutch 5 is automatically disengaged. Therefore, whatever gear the transmission 2 is put into, the wet multiplate friction clutch 5 just runs idle and the engine brake does not work, meaning that the vehicle cannot be put into a definite stop.
Further, the side brake that is ordinarily used when parking a motor vehicle is not always able to be used and is prone to certain inconveniences, for example the brake wire may freeze in cold temperatures and not be able to be unfrozen, or the wire that activates it may snap.
For these reasons, methods such as forming a multiplate disc-type countershaft brake on the end of the countershaft 29 of the transmission 2, and restricting the torque of the countershaft 29 so as to enable in-gear parking, have been considered. However, under this method a linking hole linking to the wet multiplate friction clutch 5 is opened to the casing 30 of the transmission 2 and it is necessary to extend the end of the countershaft 29 to the wet multiplate friction clutch 5. Therefore, certain disadvantages arise, for example gear oil inside the transmission 2 may leak out from the linking hole, and it is also necessary to secure a space in which to form the multiplate disc-type countershaft brake on the wet multiplate friction clutch 5. In addition, when a gear parking brake function is imposed on the countershaft brake, due to the relationship between the transmission""s reduction gears it is necessary to ensure that the brake power is greater than the reduction gears (about 1.8 times), and this requires countershaft brakes to be larger and take up a greater space.
Therefore, the present invention was designed to effectively resolve these sorts of problems. An object of the present invention is to provide a new gear parking brake of a power transmission device that enables in-gear parking to be easily achieved.
In order to resolve the above-mentioned problems, as shown in claim 1, the present invention is a gear parking brake of a power transmission device having a fluid coupling positioned on the engine side and a friction clutch positioned on the transmission side, arranged in series, comprising a brake mechanism on an input shaft of the above-mentioned transmission, which activates at the time of parking. Accordingly, even though the friction clutch becomes disengaged when the engine is stopped, the torque of the transmission""s input shaft at the time of parking can be restricted, thereby enabling so-called in-gear parking.
More specifically, as shown in claim 2, the above-mentioned brake mechanism comprises: a friction plate that revolves together with the input shaft; a pressure plate provided on the transmission housing so as to move towards and away from the friction plate; and a pressure piston provided on the transmission housing side so as to pressure weld the friction plate to the pressure plate. In other words, when the friction clutch becomes disengaged when the engine stops, the friction plate and the pressure plate are welded together by the pressure piston and the torque of the transmission""s input shaft is restricted, thereby enabling in-gear parking when the transmission has been put into any gear.
Further, as shown in claim 3, the brake mechanism may comprise a brake drum integrally formed on the transmission""s input shaft; a brake band provided so as to be able to expand and contract, and to encircle the circumference of the brake drum; and a tightening mechanism to shorten the diameter of the brake band and tighten the brake drum. In other words, when the engine is stopped, the transmission""s input shaft is tightened by the brake band and the rotation is controlled, thereby enabling in-gear parking when the transmission has been put into any gear.
Further, as shown in claim 4, if the brake drum is integrally formed in the clutch center of the friction clutch, which is formed on the input shaft of the transmission, it can be easily mounted without requiring a particularly large amount of space.
Further, as shown in claim 3, when one end of the brake band is attached to the clutch housing of the friction clutch and the other end is positioned on the brake drum, simply by pressing that end to the brake drum using the tightening mechanism, the brake band""s diameter can easily be shortened and the brake drum can be securely tightened.
Further, as shown in claim 4, by forming this tightening mechanism from a hydraulic pump that is activated by the engine""s power and a piston that is moved up and down by the oil pressure of this hydraulic pump and shortens the brake band""s diameter, at the time that the engine is stopped and the oil pressure drops, the piston projects out so as to shorten the diameter of the brake band, thereby enabling automatic activation of the mechanism simultaneous with the engine stopping, without requiring any special operation.